FIG. 36 is a general schematic configuration of a conventional image forming apparatus. As shown in FIG. 36, laser light output from a semiconductor laser unit 1009 is scanned by a rotating polygon mirror 1003, and forms an optical spot on a photosensitive element 1001, which is a medium to be scanned, via a scanning lens 1002. An electrostatic latent image is formed on the photosensitive element 1001 by exposing the photosensitive element 1001. At this time, a photodetector 1004 detects the laser light on each line.
A phase synchronization circuit 1006 generates a pixel clock whose phase is synchronized with a detection signal of the photodetector 1004, for each line, based on a clock generated by a clock generation circuit 1005. The phase synchronization circuit 1006 supplies the generated pixel clock to an image processing unit 1007 and a laser drive circuit 1008.
The semiconductor laser unit 1009 controls the emission time of the semiconductor laser, based on image data generated by the image processing unit 1007, and the pixel clock whose phase is adjusted by the phase synchronization circuit 1006 for each line. Accordingly, the semiconductor laser unit 1009 controls a forming position of the electrostatic latent image on the photosensitive element 1001.
In such a scanning optical system, the fluctuation of scanning speed results in the fluctuation of the image, thereby deteriorating the image quality. In particular, in a color image, color shift occurs due to the positional misalignment of dots of each color in the main scanning direction, thereby deteriorating color reproducibility and resolution. Accordingly, it is necessary to correct the fluctuation in scanning speed to obtain a high-quality image.
The fluctuation (error) in scanning speed may be broadly classified as follows:
(1) Error of Each Surface of Polygon Mirror (for Each Scanning Line) (Hereinafter, Appropriately Referred to as “Error of Each Surface”)
A factor for the fluctuation of the scanning speed includes the fluctuation of distance from a rotation axis of a deflector such as a polygon mirror to a deflection reflection surface (in other words, eccentricity of the polygon mirror), and the irregularity of each surface of the polygon mirror. This type of error is an error with a periodicity of few lines (for example, the number of lines corresponding to the number of surfaces of the polygon mirror).
(2) Error Due to Fluctuation in Average Scanning Speed
The average scanning speed is an average speed for scanning each surface of the polygon mirror. A factor for the fluctuation in the scanning speed includes the fluctuation of rotation speed of the polygon mirror, and the fluctuation of the scanning optical system due to various environmental fluctuations such as temperature, humidity, and vibration. When the temperature or the like varies, the oscillation wavelength of the semiconductor laser, which is a light source, changes. Accordingly, the scanning speed sometimes varies due to the chromatic aberration of the scanning optical system. This type of error varies relatively slowly.
For example, in a multi-beam optical system, such as a semiconductor laser array that includes a plurality of light sources and performs simultaneous scanning with a plurality of light beams with a common scanning optical system, the following scanning speed fluctuation occurs.
(3) Error of Each Light Source
The main factors in the error of each light source include a difference between oscillation wavelengths of the light sources, and the fluctuation of the scanning speed depending on the chromatic aberration of the scanning optical system. Because the oscillation wavelength varies depending on each light source, the error described in (2) may differ from one light source to another. The scanning speed of the beam also differs by the assembling accuracy of the light sources.
In a multi-color image forming apparatus (referred to as “tandem-type”) including a plurality of photosensitive bodies and scanning optical systems, the scanning speed difference between the scanning optical systems described below, significantly influences the image quality.
(4) Error of Each Scanning Optical System
The main factor in the error of each scanning optical system includes poor manufacturing accuracy, poor assembling accuracy, and deformation and the like with the elapse of time, of the parts of the scanning optical system. Due to the different light sources, the error described in (3) will also occur. Because the average scanning speed is different, the errors described in (1) and (2) also occur, respectively.
In such image forming apparatuses, some apparatuses commonly use a part of the scanning optical system. However, even in such an event, each optical path from the light source to a photosensitive element, which is a medium to be scanned, is different. Accordingly, the error occurs in each scanning optical system.
To correct the error of the scanning speed, for example, the frequency of the pixel clock may be changed based on the scanning speed (for example, refer to Patent Document 1 (Japanese Patent Application Laid-open Publication No. 2001-183600)). Here, the frequency of the oscillator that generates a pixel clock is controlled, so that the number counted by the pixel clock from the start to the end of the scanning may be a predetermined value (so-called phase locked loop (PLL) control).
However, in such a conventional technology, the frequency of the reference clock that performs phase comparison is the frequency of one line. Accordingly, the frequency is extremely low (a few thousandths to a few ten thousandths) compared with the frequency of the pixel clock that oscillates. Accordingly, sufficient PLL open loop gain cannot be obtained, whereby sufficient control accuracy cannot be obtained.
Because the conventional image forming apparatus is fragile against external disturbance, and the frequency of the pixel clock tends to vary. Accordingly, it is not possible to generate an accurate pixel clock. To correct the error of each surface, a control voltage of a voltage controlled oscillator (VCO), which is an oscillator, is changed for each scan. Accordingly, it takes some time until the pixel clock starts to oscillate stably.
The error of the scanning speed may also be corrected, by controlling the phase of the pixel clock based on the generated high frequency clock (for example, refer to Patent Document 2 (Japanese Patent Application Laid-open Publication No. 2004-262101)). Here, the phase of the pixel clock is controlled, so that the number counted by the high frequency clock from the start to the end of the scanning may be a predetermined value.
The high frequency clock is accurate, because it is generated from a reference clock generated by an accurate oscillator such as a crystal oscillator. Because the phase of the pixel clock is controlled based on the high frequency clock, the control accuracy of the pixel clock will also be improved.
However, in such a conventional technology, the error of the scanning speed is corrected by appropriately controlling the phase of the pixel clock. To do so, phase control data for one scanning line needs to be generated. In addition, to reduce a local deviation due to the phase change of the pixel clock, in other words, to generate a highly accurate pixel clock, high resolution phase control is necessary. Accordingly, the phase control data will be increased.
It is not easy to generate the phase control data quickly and accurately, and an extremely high speed control circuit is required to perform real-time control. Accordingly, it is not easy to perform the conventional technology. The phase control data also needs to be generated for each surface, to correct the error in each surface. Accordingly, a large amount of phase control data needs to be generated and stored to perform highly accurate correction. Accordingly, it is not easy to perform the conventional technology.
The scanning speed also varies as the following, while scanning one line, due to the accuracy error and the assembly error of the units in the scanning optical system.
(5) Nonlinear Error
FIG. 37 (a) is an example of a nonlinear error in the scanning speed of one line. A horizontal axis x is a position of a scanning line, and a vertical axis is a scanning speed V(x) corresponding to the position x. A dashed-dotted line Vavg is an average of the scanning speed of one line. When the scanning speed varies in this manner, a deviation Δ is generated as shown in FIG. 37 (b). The deviation Δ is a deviation from the ideal value obtained by scanning at a constant speed.
The deviation Δ represents a misalignment of dots, and degrades image quality. If scanning is performed in the direction towards the position X1 from X2, in FIG. 37, the deviation Δ from the ideal value is as shown by the dotted line. Accordingly, in particular, if the scanning is performed in both directions, in the scanning optical system in which an asymmetric misalignment is generated about the scanning center, the color shift is increased and the image quality degrades significantly. The amount and distribution of the nonlinear error may differ in each surface, depending on the accuracy of each surface of the polygon mirror. The error also varies in each scanning optical system.
To correct the nonlinear error in the scanning speed, the frequency of the pixel clock may be modified and corrected, corresponding to the position in the scanning line (for example, refer to Patent Document 3 (Japanese Patent. Application Laid-open Publication No. 2000-152001)).
However, in such a conventional technology, the center frequency of the pixel clock is generated in a similar manner to the conventional one. Accordingly, the accurate pixel clock as described above cannot be generated, and it is difficult to sufficiently correct the pixel clock. Consequently, it is not sufficient for obtaining a high quality image.
To solve such problems, there is a method of generating a pixel clock for correcting any scanning speed errors and nonlinear errors that would occur as described above in (1) to (5) with high accuracy (for example, refer to Patent Document 4 (Japanese Patent Application Laid-open Publication No. 2006-305780) and Patent Document 5 (Japanese Patent Application Laid-open Publication No. 2007-229932)).
However, in the conventional technologies disclosed in the Patent Documents 4 and 5, a relatively high frequency jitter (for example, jitter generated in several tens cycles) may remain in the rotation fluctuation (jitter), if the rotation speed of the polygon mirror is accelerated to increase the speed of the device.
Accordingly, in the above conventional technologies, high speed pull-in is performed by controlling and reducing the errors in all the surfaces, until the frequency of the pixel clock of each surface falls within a predetermined error range. After the frequency of the pixel clock of each surface fell within a predetermined error range, the error between the surfaces is lowered, by separately controlling each surface.
However, when the frequency of the pixel clock of each surface fell within a predetermined error range, the control is performed by obtaining an error of each surface (once every time the polygon mirror rotates), and by using the obtained error. Accordingly, the sampling frequency is reduced, thereby reducing the gain.
For example, if the polygon mirror has six surfaces, and the error is sampled in all the surfaces, the gain is reduced to a sixth, compared with before when the frequency of the pixel clock of each surface fell within a predetermined error range.
In the conventional technologies, the control bandwidth cannot be increased to stably control a system that, has a useless time of one sampling time (for example, for the stable control, the control bandwidth can only be increased up to about a few tenth to a tenth of the sampling frequency). Accordingly, there is a problem that high frequency jitter of several tens cycles cannot be suppressed sufficiently.
The present invention has been made in view of the above circumstances and intended to provide a pixel clock generator and an image forming apparatus that can increase the control bandwidth, even if the speed of the device is increased, and that can sufficiently suppress high frequency jitter.